As it is known, the more economical operation of recently available fast-working computers necessitates rapid recordal of the computer output data. Such output data are recorded mainly by line printers, but the significance of microfilm operated output systems is always increasing. The most widely used fast line printers are either of the mechanical or of the electrostatic types. The maximum speed of the faster electrostatic line printers is about 6000 lines/min. These line printers are very expensive and their manufacture is complicated. In microfilm systems the characters are displayed on the screen of a cathode ray tube and from here the characters are imaged by an optical system with a large aperture to the recording microfilm. The speed of such systems can be as high as 30,000 lines/min.
Because of their different operational principles the character generators of the two above mentioned systems cannot be interchanged. Laser operated character generators, besides their high speeds (about 50,000 to 150,000 lines/min), have the great advantage that they can be used both for line printers and for microfilm outputs; this changeover requires only the simple replacement of their output optics.
There is an apparatus known (for instance produced by Siemens (AG), in which the multi-directional deflection of a laser beam is carried out by a rotating polygon-shaped mirror, while the intensity modulation and the generation of the vertical column of a character point matrix are performed by a multi-channel acousto-optical modulator.
The operation of the multi-channel modulator is based on the selectivity of the acousto-optical Bragg diffraction. More particularly when several oscillator output signals with different frequencies are coupled to a single ultrasound transducer of an acousto-optical cell simultaneously, there will be as many deflected outgoing laser beams as there are oscillators. These deflected beams can be spatially separated from each other and their intensity can easily be modulated almost independently by changing the output powers of the oscillators. The multi-channel modulator requires the use of an amplitude adder which is a complicated device and a high power linear amplifier. The controlling electronic circuitry become particularly complicated and expensive when the number of the channels, for improving the character quality, exceeds about 10 or 20.
In laser-operated line printer produced by IBM the intensity modulation is effected magneto-optically the line-directional deflection is carried out by a rotating polygonal mirror, and the generation of the characters occurs within the point matrix by lines i.e. on a miniraster. This makes the synchronization complicated and expensive, while due to the sequential mode of operation the bandwidth of the modulator should be high.
In a line printer produced by the RCA corporation the generation of the characters occurs in minirasters (altogether 19 pieces). This has the same drawbacks as mentioned in connection with the IBM system.
There are also known systems designed for producing microfilms such as the laser-operated character generators of Datalight Inc., of Stromberg Datagraphix Inc., and of the 3M Co., the operation of these systems is similar to that of the Siemens system with the exception that the line-directional deflection is carried out by a swinging mirror. The characters are composed on a 7.times.5 or 9.times.7 point matrix.
Due to the use of an adder-type multi-channel modulator, the number of raster points in the column direction (7 and 9) within the point matrix cannot be noticably increased further economically. The further improvement of the quality of the characters can be reached only by generating miniraster based characters which is both complicated and expensive.